Depends. Some considerations:
Hopefully there will be stage lighting. You shouldn't need a flash. If it's a low-budget kind of thing, the stage lights might be crap - but an on-camera hotshoe flash will detract from the overall "feel" of the show unless it's used very, very judiciously. Mort importantly, the flash will detract from the performance for the other audience members. It's EXTREMELY rare that I use flash at any sort of show where there are other audience members.
Watch your shutter speed; don't let it go too slow or you'll get objectionable motion blur. 1/100 is just about the absolute minimum I would use for a performance; if they're energetic then you may need to go higher - 1/160, 1/200, maybe even 1/400 if they're jumping all around.
The 5D3 should focus pretty reliably, so you can get away with a relatively wide aperture. I usually use my 70-200 f/4L wide open, and it's VERY rare for me to lose a shot due to missed focus. Sometimes I use one-shot, sometimes I use AI servo - depends on the performance and my mood. Depends on how confident you are with your gear and your lenses.
Don't be afraid to crank up the ISO in order to get a proper exposure. The 5D3 can handle it, especially with some basic use of noise reduction in post. Here's an example of what can be done with the 5D3 at ISO 12800:
https://photography-on-the.net …/showthread.php?t=1360713
Careful with your metering modes if you're using any of the automatic modes. If it's a spotlight-illuminated singer against a black background, many of the metering modes will let the singer blow out in order to average out the overall exposure. Even at moderate ISO, blown-out skin is hard to recover. At high ISO, blown-out skin is impossible to recover. Let the background fade to clipped black if you must, but DON'T let the skin blow out. Don't be afraid to turn on the blinkies and chimp, to make sure you're not clipping anything important.
As far as what lens to use? Depends on your style, your location, and the nature of the show. If you have free run of the place and can get up close to the stage, you have more flexibility in lens/FL choice. My style doesn't lend itself well to up-close shots with a wide-angle, though some people can pull it off with great results. I prefer to stand back a little bit and use the 70-200 to get closeups of faces and expressions. Mix in a few shots with a wide-angle to take in the whole venue, but don't stick to the wide-angle the whole time.
Caveat: If lighting is at a premium, you might have to put on the 35mm f/1.4, open it all the way up, and just do the best you can with what you got. A few weeks I shot a show in a little bar, where the only front lighting was from a jukebox on the other side of the room, and some back lighting from a couple track lights at the rear of the stage. I was at 1/125, f/1.4, and ISO25600 for most of the show.
